This project is designed to provide information on the ranges and distributions of the electrophysiological and morphological characteristics of alpha motoneurons and of the interrelated mechanical, histochemical and morphological properties of the muscle fibers innervated by them (i.e., the muscle unit) in various hindlimb muscles in the cat. Methods used include intracellular recording and stimulation, measurement of mechanical properties of muscles and individual muscle units, neuroanatomical techniques of intracellular staining with horseradish peroxidase, along with conventional and computer- aided methods for reconstruction of extensive neuronal structures from serial histological sections, and computer modeling and data processing. In some experiments, motor unit populations in normal animals are compared with those in animals after various conditioning treatments. Studies of alpha motoneuron properties are included in this project when they are related importantly to the type of muscle unit innervated by the studied cells.